Phantom is the abbreviation of biological tissue mimicking model. Currently, there are various ultrasonic phantoms, most of which are for applications of ultrasonic diagnosis, and for microwave hyperthermia and HIFU (High Intensity Focused Ultrasound) studies. However, the purpose of a phantom for ultrasound therapy is completely different from that of a phantom for ultrasound diagnosis. HIFU is applied in the treatment of tumor by causing coagulate tissue necrosis. Ultrasonic phantom for diagnosis is typically opaque, and cannot generate a macroscopic coagulation necrosis after HIFU radiation, although it has acoustic properties similar to those of a soft tissue.
HIFU is a newly immerging technology for treating tumor by a non-invasive, conformal thermal ablation method guided by ultrasound or MRI (Magnetic Resonance Imaging). During clinical treatment using a HIFU system, the HIFU system must be quality tested, i.e., effectiveness, safety, and reliability, wherein the most crucial test is the evaluation of the focusing performance of the HIFU treating applicator. The focusing performance of the HIFU treating applicator on the tissues is directly related to effectiveness, safety, and reliability of HIFU during clinical application. However, it is thus far difficult to evaluate the focusing performance of the HIFU treating applicator with a non-standardized biological tissue, and compare focusing performances of different HIFU treating applicators. Therefore, it is very important to establish a standardized, reusable transparent tissue mimicking phantom.
At present, the phantoms used in HIFU studies both in China and abroad are transparent tissue mimicking phantoms formed by a mixture mainly comprising polyacrylamide and protein (bovine serum or fresh egg white) that serves as a temperature-sensitive indicator. Upon HIFU radiation, protein undergoes thermal denaturation, and the appearance of the phantom turns cloudy or white, thereby revealing the focal region of the focused ultrasound (See Fa-qi LI, Ping M A, Xiao-qin K O U, et al, A transparent tissue-mimicking phantom for evaluating the focusing performance of HIFU, Chinese Journal of Medical Imaging Technology, 2006, 22(8): 1261-1265). However, this protein phantom has many disadvantages:                (1) The cloudy region is not stable, the boundary is not well-defined, and the properties are much inferior to those of BFR (Biological focal region, i.e., the coagulation necrosis region visible by naked eyes and confirmed by a microscope, which appears at the location of the acoustic focus in the biological tissue where HIFU radiation is applied. Reference: Yong-chang ZHOU, Wan-xue GUO. Ultrasound Medicine [M]. Fourth Edition. Beijing: Science Press, 2002.1780.) produced in biological tissues, therefore, this protein phantom is not very suitable for studying the focusing performance of HIFU in biological tissues.        (2) If bubbles are contained therein, the white region will become irregular.        (3) As a temperature-sensitive indicator, once protein is denatured by heat, its appearance then turns white and cannot be restored. Therefore, it can only be used once associated with high cost.        (4) Although additives such as preservatives and foam inhibitors can be incorporated therein, the main material (i.e., protein) is prone to deterioration, and therefore, the stability of the phantom and the effectiveness to eliminate the impact of bubbles in the preparation process are limited.        (5) As the source of transparent protein, bovine serum or fresh egg white cannot assure the consistency of its quality, therefore the consistency of the phantom also cannot be assured, and standardization of the phantom cannot be realized.        (6) Restricted by factors such as the nature of protein, visible change in the appearance can only be seen at a temperature of 70° C. or higher (i.e., the denaturation temperature of protein), and this temperature is higher than that at which coagulation necrosis occurs in real biological body tissues (typically 60-65° C.), and different biological body tissues may have different temperatures at which coagulation necrosis occurs. The denaturation temperature of the protein phantom is thus fixed, and the requirements of different denaturation temperatures cannot be satisfied. Therefore, a phantom that has a denaturation temperature below 70° C. and can be used at different denaturation temperatures are desired by those skilled in the art (See: Sam Howard, Jonathan Yuen, Paul Wegner, et al. Characterization and FEA Simulation for a HIFU Phantom Material. 2003 IEEE International Ultrasonics Symposium, 2003, Vol. 2: 1270-1272).        